Medical imaging produces large volumes of data in data records which, following further processing, are displayed to the user as image information for a region under examination on the examined object. The data obtained using the imaging methods frequently also contain further information which may be of value to the user. This is explained in more detail below using the example of radiographical methods, particularly of X-ray computed tomography (CT).
The result of radiographical methods, such as computed tomography, mammography, and angiography, X-ray inspection engineering or comparable methods, is first of all the representation of the linear attenuation of an X-ray along its path from the X-ray source to the X-ray detector in a projection image.
This linear attenuation is caused by the irradiated materials along the ray path, which means that the linear attenuation can also be understood as a linear integral over the linear attenuation coefficients of all of the volume elements (voxels) along the ray path.
Particularly in the case of tomographical methods such as X-ray computed tomography, reconstruction methods can be used to calculate back from the projected linear attenuation data to the linear attenuation coefficients μ of the individual voxels and hence to obtain a significantly more sensitive examination than in the case of pure observation of projection images.
To represent the linear attenuation distribution, a value which is normalized to the linear attenuation coefficient of water and is called the “CT number” is normally used instead of the linear attenuation coefficient. This is calculated from a linear attenuation coefficient μ currently ascertained through measurement and from the reference linear attenuation coefficient μH2O according to the following equation:
  C  =      1000    ×                            µ          ⁢                                          ⁢                      µ            H2O                                    µ          H2O                    ⁢                          [      HU      ]      with the CT number C in the unit Hounsfield [HU]. For water, a value CH2O=0 HU is obtained, and for air a value CL=−1000 HU. Since the two representations can be transformed into one another or are equivalent, the generally chosen term linear attenuation value or linear attenuation coefficient refers both to the linear attenuation coefficient μ and to CT value below.
The linear attenuation value for an X-ray scan cannot be used to infer the material composition of an object under examination, however, since the X-ray absorption is determined both by the effective ordinal number for the material and by the material density.
Materials or tissue of different chemical and physical composition may therefore have identical linear attenuation values on the X-ray image.
B. J. Heismann et al., Density and Atomic Number Measurements with Spectral X-Ray Attenuation Method, J. of Appl. Phys., Vol. 94, No 3, 2003, 2073-2079 and German patent application DE 101 43 131 A1 disclose a method in which at least two data records from the same region under examination are recorded with different spectral distribution of the X-ray radiation and/or X-ray detection. Using the spectral information, it is then possible to calculate the spatial distribution of the density ρ(r) and of the effective ordinal number Z(r) in the region under examination, also referred to as ρ-Z projection below, from the measurement data records. From combined evaluation of the distribution of the density and of the effective ordinal number it is possible to determine body constituents such as iodine or the like quantitatively and, by way of example, to segment out calcifications based on the ordinal number.
Further techniques for using the spectral information in the two measurement data records are known from the subsequently published documents DE 103 11 628 and DE 103 47 971, the entire contents of each of which are hereby incorporated herein by reference.
The potential opportunities for using the quantitative information contained in the data records give rise to the problem of suitable representation of the results, so that the user of the imaging system obtains the sometimes complex quantitative information in an easily diagnosable representation.
DE 101 27 573 A1 relates to a method for the combined representation of morphology and dynamics in the case of sectional-image and volumetric-image methods. The object of this method is to reduce the radiation load for a scan sequence in an imaging method based on X-ray radiation. In this context, at least two images of different phases of the corresponding tissue are recorded. Next, the at least two images are added in anatomical agreement in order to obtain the morphology of the tissue. In a further step, the difference between the images is calculated and the alterations are color coded. In this color-coded representation, changes over time can be visually detected immediately.